High speed flying hammer solenoid systems



I A ril 21, 1970 R. DERC 3,507,213

HIGH SPEED FLYING HAMMER SOLENOID SYSTEMS Filed Oct. 16, 1967 2 Sheets-Sheet z United States Patent U.S. Cl. 101-93 2 Claims ABSTRACT OF THE DISCLOSURE High speed printing apparatus in which a drum bearing a complete font of characters revolves at high speed, adjacent to a hammer, and a paper web and printing ribbon are passed between the hammer and the drum. The hammer is fired at the drum, by means of a solenoid and lever, at the instant when a desired character is opposite the hammer, thus causing the character to be printed. Damping the hammer movement after printing is achieved by energising the solenoid with a current of suitable magnitude when the hammer returns to its initial position. This is preferably achieved by a diode shunted across the solenoid winding, with a suitable resistance in the loop so formed. The solenoid may be energised from a capacitor via an SCR, and by providing a small saturable inductor in series with the diode, the SCR is turned off sharply at the end of the capacitor discharge.

This invention relates to high speed flying hammer solenoid systems.

In such systems, a hammer is held in a normal position by light retaining means such as a light spring. On energisation of an associated solenoid, the hammer is accelerated, either by direct action or by means of a lever, away from its normal position towards an operating position, the accelerating force acting over only the first part of its path and the hammer travelling or flying over the remaining part of its path substantially free of restraints. When the hammer reaches the operating position, it strikes against relatively rigid and unyielding stop means, and rebounds therefrom, returning as a result of this rebound over its original path to its normal position.

Hammers of this type are commonly used in high speed data printers of the drum type. In such a printer, there is a constantly rotating drum with, say, 160 similar circumferential bands each containing, say, the 64 characters of a font in raised form. A printing ribbon and a sheet of paper are passed past the drum, and a row of 160 flying hammer solenoid systems is placed adjacent to the drum. On energisation of a solenoid, the, associated hammer is moved rapidly to its operating position, in which it strikes the paper and printing ribbon and the drum immediately behind them. That character on the drum which happens to be at the operating position of the hammer at that time is therefore printed on the paper. Thus by energising the solenoids at appropriate instants, a row of desired characters can be printed.

In such high speed flying hammer solenoid systems, it is desirable for the system to return to its initial state as rapidly as possible after operation. Investigation of the various events which occur during an operation of the system has shown that the hammer, on rebounding from the operating position, retains a substantial part of its initial kinetic energy, and that this energy causes the hammer to bounce when it returns to its initial position. In fact, in a badly designed system it is possible for the hammer to bounce back as far as the operating position again, so that over-printing of the desired character with another may occur in a printer. Whether or not this over-printing occurs, however, there is a substantial time required for the hammer to settle into its initial position under the influence of the light restraining means after bouncing.

The object of the present invention is to diminish the tendency of the hammer to bounce. This is achieved as follows. In the known systems, the solenoid is energised by a current pulse which lasts for a time not much longer than the time during which the hammer is undergoing its initial acceleration from the normal position, the hammer of course acquiring energy from the electrical circuit at this time. By this invention, the solenoid is arranged to be energised with a current of suitable magnitude (herein termed damping current) during the time when the hammer is returning over the first part of its path (i.e. the part nearest its normal position) such that the hammer is decelerated by the solenoid and a substantial part of its energy is absorbed by the electrical circuit.

A conventional solenoid driving circuit is known in which the solenoid is connected across a capacitor via an electronic switch, the capacitor being slowly charged from a power supply and being rapidly discharged to energise the solenoid when the switch is closed.

According to a subsidiary feature of this invention, this circuit is arranged to provide the required damping current by shunting the solenoid with a diode poled so as to be non-conductive during the capacitor discharge, the current through the solenoid when the capacitor is fully discharged continuing to flow through the diode. and the resistance of the loop formed by the solenoid and the diode being chosen so that a damping current of the necessary amplitude at the appropriate time is produced. This resistance is preferably formed by the inherent resistance of the solenoid winding.

A particularly convenient form of the known circuit utilises a silicon controlled rectifier (SCR) as the switch and includes transistor means for charging the capacitor. The discharge path of the capacitor is coupled to these transistor means in such a way as to ensure that there is no charging current while the switch is turned on; this prevents the switch being fed through the transistor means and becoming latched permanently on. The current through the capacitor on discharge starts at zero and builds up to a peak roughly sinusoidally. After it has passed its peak but well before it reaches zero, the diode across the solenoid winding becomes conductive. Ideally the capacitor current should then drop sharply to zero. However, due to small impedances in the discharge path (e.g. caused by the coupling to the transistor means), the capacitor will in fact retain a small charge, and its discharge current will have a sharp drop-to just above zero followed by a long tail as it slowly falls to zero. The coupling between the discharge path and the transistor means prevents the capacitor from starting to recharge until it is fully discharged. There is thus an undesirable extension of the time required for the circuit to return to its initial state.

By a further subsidiary feature of this invention, this disadvantage is overcome by providing a small saturable inductance in series with the diode. This results in delaying the action of the diode slightly, the inductance appearing as an appreciable impedance in series with the diode until the current through it reaches the saturation value. The capacitor will therefore acquire a slight reverse charge in this period. When the current through the inductor and diode reaches the saturation value, the voltage across the inductor falls sharply to zero, and the reverse voltage on the capacitor then appears across the switch, turning it off sharply.

An embodiment of the invention incorporated in a high speed data printer will now be described with reference to the accompanying drawing in which:

FIG. 1 shows the mechanical arrangement of the solenoid and hammer;

FIG. 2 shows the electrical driving circuit of the solenoid;

FIG. 3 is a graph of the current through the solenoid plotted against time;

FIG. 4 is a graph of the voltage across the solenoid plotted against time; and

FIG. 5 is two graphs showing the lever and hammer positions plotted against time.

FIGS. 3 to 5 have a common time scale.

Referring to FIG. 1, a solenoid has its pole faces 11 facing one end of a lever 12 which is pivoted on a pivot 13. The lever 12 is normally urged, by means of a light spring 14, in the anti-clockwise direction (as seen in FIG. 1) so that its lefthand end rests against a stop 15. A hammer 16 is arranged above the lefthand end of the lever 12, being urged downwardly by a light leaf spring 18, and normally rests on a pad 17 fixed to the lever 12. The hammer 16 passes through bearings 19, which allow it to move only in the vertical direction.

A print drum 20 is arranged above the top end of the hammer 16, and has a multiplicity of types 21 formed at equal intervals around its periphery. A sheet of paper 22 and a printing ribbon 23 are arranged between the hammer 16 and the print drum 20 as shown.

In operation, the print drum 20 rotates at a uniform speed. Each in turn of the types 21 is therefore presented at the printing station, i.e. vertically above the upper end of the hammer 16. Timing and control circuitry is provided which energises the solenoid 10 at an appropriate time with a current pulse 40 as shown in FIG. 3. This results in the righthand end of the lever 12 being attracted downwardly towards the solenoid 10, and the lever 12 is therefore caused to rock rapidly in the clockwise direction until its righthand end contacts the pole faces 11 of the solenoid 10. The lefthand end of the lever 12 therefore imparts a rapid acceleration to the hammer 16, which continues to move substantially unchecked when the lever 12 is stopped by contact with the pole faces 11 of the solenoid. The hammer 16 continues to move upwards until its upper end strikes the paper 22 and printing ribbon 23, forcing these against that one of the types 21 which is above the hammer 16 at this instant. The appropriate character is thereby printed on the paper 22. This printing action occurs in a very short dwell time, during which the motion of the print drum 20 is negligible.

The movements of the lefthand end of lever 12 and of the hammer 16 are shown by graphs 41 and 42 respectively of FIG. 5, the vertical scale representing distance moved from the initial positions. The hammer 16 is in contact with the lever 12 up to the point 43, at which the lever 12 is rocked fully anti-clockwise, and the hammer then flies freely to its operating position (point 44), where the dwell occurs.

When the dwell ends, the hammer 16 rebounds from the printing drum 20 and returns downwardly at a speed comparable with its initial upwards speed. Eventually, it again contacts the lever 12 (point 45, FIG. 5), which is still rocked fully clockwise. This therefore causes the lever 12 to rock anti-clockwise and a substantial part of the initial rebound energy of the hammer 16 is transferred to the lever 12. The solenoid is arranged to be energised at this time with a damping current (46, FIG. 3), and a substantial part of the mechanical energy of the lever 12 is used in moving it against the attractive force of the solenoid. This energy appears in the electrical circuit as an increase of current, seen in FIG. 3 as the rise 47. The rebound energy of the hammer 16 is thus largely absorbed electrically, and the hammer 16 and lever 12 return rapidly and smoothly to, and are retained substantially without bouncing in, their final normal positions by means of the light springs 18 and 14 respectively, as shown by portions 48 and 49 of the graphs 41 and 42, FIG. 5. v

Referring now to FIG. 2, the drive circuit of the solenoid 10 is shown. This circuit includes a capacitor C1 which is gradually charged by means of a charging impedance in the form of a transistor Trl, and can be rapidly discharged through the winding 30 of the solenoid 10 in response to a trigger signal applied via a terminal 31 to an electronic switch in the form of a silicon controlled rectifier SCR 1. More specifically, the transistor T11 is controlled by a second transistor Tr2, whose base is connected via a diode D3 to an adjustable bias voltage source formed by voltage dividers R5 and R6. The capacitor C1 is therefore charged through resistor R1 and the emitter-collector path of transistor Trl, transistor Tr1 being turned off when its emitter voltage, i.e. the voltage on capacitor C1, equals its base voltage. The base of transistor Trl is connected to the emitter of transistor Tr2, which is connected to earth via resistors R3 and R4, Transistor Tr2 therefore acts as an emitter follower, and the base voltage of transistor Trl and hence the final charged voltage of capacitor C1 is determined by the voltage dividers R5 and R6. The resistor R4, which is of high resistance, minimises any tendency of the voltage on capacitor C1 to creep up slowly over the last part of its charging.

Capacitor C1 is connected, through diode D1, to one end of the winding 30, whose other end is connected through the silicon controlled rectifier SCR 1 to earth. When the solenoid is to be energised a positive-going input signal is applied to terminal 31, and passes over resistor R7 to the control electrode of the silicon controlled rectifier SCR 1, which is thereby turned on. Capacitor C1 therefore discharges through diode D1, winding 30 and the silicon controlled rectifier SCR 1.

Owing to the high inductance of the winding 30, the current flowing through it cannot rise instantaneously. In order to prevent the silicon controlled rectifier SCR 1 from turning off after the input pulse has ended and before the current through winding 30 has built up appreciably, resistor R8 is provided in shunt across the winding 30, this resistor having a relatively high resistance but passing enough current, immediately following the input pulse, to hold the silicon controlled rectifier SCR 1 conductive. A diode D2 is connected as shown, the voltage drop across the diode D1 causing a small current to flow in the series path of the resistor R3 and diode D2 shunting diode D1. Diodes D1 and D2 are silicon and germanium diodes respectively. The voltage drops across the resistor R3 and the diode D2 are therefore about 0.5 v., each, and these voltages ensure that the transistors Tr1 and Tr2 are turned off during the discharge of capacitor C1. As the discharge of capacitor C1 continues, the current in solenoid winding 30 builds up as shown by the pulse 40 in FIG. 3. The fall in the trailing edge of this pulse 40 is due to the combined effects of the small resistances in the discharge path of the capacitor C1 and the changing inductance of the solenoid winding 30 as the lever 12 is attracted towards it.

Eventually, the voltage on capacitor C1 falls to zero as shown in FIG. 4. At this point further movement of the voltage at the junction of diode D1 and the winding 30 in the negative direction is prevented by the diode D4 becoming conductive. (The saturable inductor L1 will be neglected for the moment.) The current through the silicon controlled rectifier SCR 1 falls to Zero, and the silicon controlled rectifier therefore turns off, while the current through the solenoid winding 30 continues to flow around the loop completed by diode D4.

The current in this loop decays exponentially at a rate determined by the resistance and inductance of the winding 30. The resistance of winding 30 is chosen, by a suitable choice of the wire used for the winding, so

that the current decays to an amplitude suitable to effect the requisite absorption of energy when the lever 12 is rocked anti-clockwise by the hammer 16 returning from its operating position. As the lever 12 moves away from the solenoid 10, the inductance of the Winding 30 decreases and the current through it increases proportionally, as shown by the rise 47 in FIG. 3. This represents an increase in the electrical energy stored in the winding, which is proportional to the product of the inductance and the square of the current. This energy is gradually dissipated by the resistance of winding 30 as the current decays; this decay is faster than the decay before the rise 47, owing to the reduced inductance of the solenoid as the lever 12 moves away from it.

Simultaneously with this process, the transistors Trl and Tr2 turn on again once the discharge current of capacitor C1 through diode D1 has fallen to zero, and the capacitor is charged up again in readiness for the next operation of the system.

The effect of the small saturable inductor L1 will now be considered. In order to recharge capacitor C1 and return the circuit to its initial state as soon as possible, it is desirable to switch the silicon controlled rectifier SCR 1 off as soon as possible after the main discharge of capacitor C1. If the winding 30 were not shunted by diode D4, the capacitor C1 and winding 30 would form a relatively lightly damped oscillatory circuit, and the discharge of the capacitor would be smoothly followed by a charging in the opposite direction, the current in winding 30 falling smoothly to zero at the same time. The provision of diode D4 alone prevents the voltage across winding 30 going negative at all. The inclusion of a suitable impedance in series with diode D4, however, allows the voltage across winding 30 and hence the voltage on capacitor C1 to go slightly negative. The current through the inductor L1 which forms this impedance soon rises to the saturation value, and the voltage across it then falls sharply to zero. This leaves the capacitor C1 with a small negative voltage, which therefore appears across the silicon controlled rectifier SCR 1 and the diode D1, which are effectively in series.

The diode D1 is a high current diode and therefore has a relatively high capacitance so that the voltage across it cannot change rapidly from the very low voltage which it had during the discharge of capacitor C1. Most of the negative voltage of capacitor C1 therefore appears as a reverse voltage across the silicon controlled rectifier SCR 1, which is switched olf rapidly and reliably as a result.

The waveform of the voltage across the winding 30 therefore exhibits a small negative pulse 50, as shown in FIG. 4, the trailing edge of which is effective to turn the silicon controlled rectifier SCR 1 011. By choosing the magnitude of the saturation current of the inductor L1, the magnitude of the pulse can be adjusted to total of solenoid systems are provided in the printer, and a single potentiometer R6 is provided common to all solenoid systems. Thus individual adjustment of each system can be achieved by adjusting the potentiometer R5, while potentiometer R 6 permits the common adjustment of all solenoid systems.

I claim:

1. A high speed flying hammer solenoid system, comprising:

a hammer capable of travelling between a normal position and an operation position; light retaining means for holding the hammer in its normal position; a solenoid [coupled to] operable to move the hammer;

solenoid drive means for energising the solenoid with a short current pulse; the energisation of the solenoid accelerating the hammer towards the operating position over the first part of its path, the hammer then moving substantially freely to the operating position and rebounding towards the normal position;

and damping current supply means for energising the solenoid during the time when the hammer returns over that part of its path where it was accelerated, with a damping current effective to cause deceleration of the hammer and absorption of a substantial part of its kinetic energy;

wherein the solenoid drive means comprises; an electronic switch connected in series with the solenoid to form a series circuit;

a capacitor having the series circuit connected across it and dischargeable through the series circuit;

and a power supply and a charging impedance connected in series across the capacitor;

and wherein said damping current supply means comprises; a diode connected across the solenoid and poled so as to be non-conductive during the discharge of the capacitor; and a resistance connected in the loop formed by the diode and the solenoid.

2. A system according to claim 1, wherein the charging impedance is a transistor which is turned off during discharge of the capacitor, and the electronic switch is an SCR, and further including a small saturable inductor connected in series with the diode.

References Cited UNITED STATES PATENTS 3,045,148 7/1962 McNulty et a1. 123-148 XR 3,072,045 1/1963 Goin -l01--93 3,124,724 3/1964 Mihalek 317-148.5 3,140,428 7/1964 Shepard 317-1485 3,183,830 5/1965 Fisher et a1. 10193 3,199,650 8/1965 Brown et al 101-93 XR 3,335,659 8/1967 Schacht et al. 101-93 ROBERT E. PULFREY, Primary Examiner C. D. CROWDER, Assistant Examiner U.S. Cl. X.R. 

